1. Field of the Invention
The present invention relates to a radio receiver and more particularly, to a radio receiver for receiving a spread spectrum signal bearing an information or message the bandwidth of which has been spread out to a wider bandwidth in a transmitter.
2. Description of the Prior Art
When two or more jamming waves whose frequencies are outside the frequency range of a wanted signal wave are received by a radio receiver used for mobile phones or selective calling, the signal wave may be affected by the jamming waves. This is because the two or more jamming waves may be mixed together due to the nonlinear characteristics of the receiver, thereby generating a mixed jamming wave whose frequency is inside the frequency range of the wanted signal wave. This problem has been known as the "inter-modulation (TM)" and considered as an important factor that affects the characteristic or performance of the receiver.
If the inter-modulation occurs, in spite of the sufficient strength or intensity of the electric field of the wanted signal wave, the bit error rate will degrade in digital communication and the SINAD will degrade in analog communication. Here, the abbreviation "SINAD" mean the ratio of Signal, Noise, and Distortion to Noise and Distortion.
To prevent the received signal strength from degrading due to the TM-induced jamming wave, various improved receivers of this sort have been developed and reported.
An example of the conventional radio receivers is shown in FIG. 1, which is disclosed in the Japanese Non-Examined Patent Publication No. 5-335857 published in 1993.
In FIG. 1, a signal wave that has been transmitted from a base station (not shown) is received by an antenna 101. The signal wave thus received is amplified by an Radio-Frequency (RF) amplifier 102 and then applied to a RF filter 104 through a variable attenuator 103. The bandwidth of the signal wave is limited by the RP filter 104. The signal wave whose bandwidth is limited is then divided into the In-phase (I) component and the Quadrature (Q) component, and the I and Q components are applied to frequency mixers or converters 105 and 106, respectively.
The frequency mixer 106 is applied with a load signal of a specific local frequency outputted from a local oscillator 107. On the other hand, the local signal is applied to a phase shifter 108 and is phase-shifted by (.pi./2), resulting in a phase-shifted local signal. The phase-shifted local signal is applied to the frequency mixer 105.
In the frequency mixer 105, the I component of the signal wave is frequency-mixed with the phase-shifted local signal. The frequency-mixer I component of the signal wave is then applied to a baseband filter 109, resulting in the I component in the baseband frequency range. The I component of the signal wave in the baseband frequency range is further applied to a demodulator 111.
In the frequency mixer 106, the Q component of the signal wave is frequency-mixed with the local signal. The frequency-mixed Q component of the signal wave is then applied to a baseband filter 110, resulting in the Q component in the baseband frequency range. The Q component of the signal wave in the baseband frequency range is further applied to the demodulator 111.
The I and Q components of the signal wave are digitally demodulated in the demodulator 111 to thereby recover a digital information signal transmitted by the received signal wave. The digital information signal is then applied to a controller circuit 112.
In the controller circuit 112, the frame synchronization signal included in the demodulated information signal is detected. If the frame synchronization signal is not detected within a specific period of time, the controller 112 treats the demodulated information signal as de-synchronized and informs a gain controller 113 of the de-synchronization. To respond this information, the gain controller 113 controls the attenuation amount of the variable attenuator 103.
Thereafter, the same procedure as above is repeated until the frame synchronization signal included in the demodulated information signal is detected in the controller circuit 112. When the frame synchronization signal included in the demodulated information signal is detected in the controller circuit 112, the attenuation amount of the variable attenuator 103 is returned to its original value.
With the conventional radio receiver shown in FIG. 1, as described above, the attenuation amount of the variable attenuator 103 is estimated in advance. Then, if the frame synchronization signal is not detected within a specific period of time in spite of the sufficient strength or intensity of the electric field of the wanted signal wave, the controller 112 treats the demodulated information signal by the demodulator 111 as de-synchronized (i.e., occurrence of the inter-modulation) and controls the attenuation amount of the variable attenuator 103 through the gain controller 113.
Thus, the jamming waves induced by the inter-modulation that occurs in the subsequent stages to the variable attenuator 103 are able to be suppressed.
Another example of the conventional radio receivers is shown in FIG. 2, which is disclosed in the Japanese Non-Examined Patent Publication No. 7-106993 published in 1995.
In FIG. 2, a signal wave that has been transmitted from a base station (not shown) is received by an antenna 201. The received signal is then applied to a RF switch 203 through a branching filter 202. The RF switch 203 sends the received signal to a RF filter 205 directly or through a RF amplifier 204 according to a control signal outputted from a logic circuit 215.
When the received signal is sent to the RF filter 205 through the RF amplifier 204, the received signal is amplified by the RF amplifier 204 and then, it is inputted into the RF filter 205. On the other hand, when the received signal is directly sent to the RF filter 205, then received signal is directly inputted into the RF filter 205 without amplification. The RF filter 205 removes the unnecessary waves outside the frequency range of the received signal.
A first frequency converter or mixer 206 frequency-converts the received signal using a first local signal generated by a first local oscillator 216, thereby producing a first Intermediate-Frequency (IF) signal. The frequency range of the first IF signal is limited by an IF filter 207.
A second frequency converter or mixer 208 frequency-converts the received signal thus frequency-range-limited using a second local signal generated by a second local oscillator 217, thereby producing a second IF signal. The second IF signal is then amplified by an IF amplifier 209. The amplified second IF signal is inputted into a field-strength detector circuit 210 and a demodulator circuit 211.
The demodulator circuit 211 demodulates the amplified second IF signal thus inputted and outputs a demodulated signal to a line-quality detector circuit 212 and an audio codec 213. The audio codec 213 converts the demodulated signal to an audio signal and drives a speaker 214 according to the audio signal.
The field-strength detector circuit 210 outputs a high-level (H) output signal to a logic circuit 215 when the electric-field strength of the received signal wave is equal to or greater than a specific level, and a low-level (L) output signal to the logic circuit 215 when the electric-field strength of the received signal wave is less than the specific level.
The line-quality detector circuit 212 detects the line quality and outputs a high-level (H) output signal to the logic circuit 215 when the line quality is greater than a specific level, and a low-level (L) output signal to the logic circuit 215 when the line quality is equal to or less than the specific level.
The logic circuit 215 controls the switching operation of the RF switch 203 according to the output signals of the field-strength detector circuit 210 and the line-quality detector circuit 212.
Specifically, when the output signal of the field-strength detector circuit 210 is in the L level, the logic circuit 215 controls the RF switch 203 so that the output of the branching filter 202 is sent to the RF amplifier 204, which is independent of the level of the output signal of the line-quality detector circuit 212.
When the output signals of the field-strength detector circuit 210 and line-quality detector circuit 212 are in the H level, the logic circuit 215 controls the RF switch 203 so that the output of the branching filter 202 is sent to the RF amplifier 204.
When the output signal of the field-strength detector circuit 210 is in the H level, and the output signal of the line-quality detector circuit 212 is in the L level, the logic circuit 215 controls the RF switch 203 so that the output of the branching filter 202 is directly sent to the RF filter 205.
With the conventional radio receiver shown in FIG. 2, as described above, when the electric-field strength of the received signal wave is large enough (i.e., equal to or greater than the specific level) and at the same time, the line quality is excessively degraded (i.e., equal to or less than the specific level), the logic circuit 215 controls the RF switch 203 so that the received signal wave outputted from the branching filter 202 is directly sent to the RF filter 205 while bypassing the RF amplifier 204.
Thus, if a plurality of jamming waves exist, the received signal wave is inputted into the first frequency converter 206 without amplification. This means that the jamming signal induced by the inter-modulation or cross-modulation in the first frequency converter 206 is able to be suppressed.
However, the above-described conventional radio receivers shown in FIGS. 1 and 2 have the following problems.
With the conventional radio receiver shown in FIG. 1, as described above, the sensitivity degradation of the receiver is judged by detecting the frame synchronization signal. Therefore, if this radio receiver is applied to the communicating system using a plurality of channels whose transmission powers are different from one another, the sensitivity degradation of the receiver is not always suppressed.
For example, in the mobile communication system regulated by the Telecommunications Industry Association (TIA) as the Interim Standard 95 (IS95) in north America, three channels, i.e., the pilot channel, the traffic channel, and the synchronization channel are used. The pilot channel (PLT) is used to transmit the synchronization signal with the spreading code in the transmitter. The traffic channel (TRF) is used to transmit the information to be transmitted, which is scrambled. The synchronization channel (SYNC) is used to transmit the synchronization signal with the frame in the transmitter and the releasing or removing signal for the scramble in the traffic channel. The transmission power of these three channels are set to satisfy the relationship of PLT:TRF:SYNC=3:1:1.
In this mobile communication system, the pilot channel signal is transmitted with a higher electric power than those of the traffic and synchronization channel signals. Therefore, even if the receiver is synchronized with the spreading code in the transmitter through the pilot channel, the synchronization channel may be affected by the inter-modulation or cross modulation. This means that the scramble in the traffic channel is unable to be removed.
As a result, even if the receiver is able to be synchronized with the spreading code in the transmitter through the pilot channel, the sensitivity degradation (or, the electric-field strength lowering) of the receiver may take place due to the inter- or cross-modulation. This is a first problem of the conventional radio receiver shown in FIG. 1.
A second problem of the conventional radio receiver shown in FIG. 1 is that the demodulator 111 needs to have a very wide dynamic range. This is caused by the fact that the electric-field strength of the received signal wave is controlled or adjusted by the variable attenuator 103 and therefore, the level of electric-field strength of the demodulated signal inputted into the demodulator 111 is not kept in a specific level.
For example, if the attenuation amount of the variable attenuator 103 is increased to suppress the effect of the jamming wave, the electric-field strength of the received signal wave will be lowered compared with the case where no jamming wave exists.
A problem of the conventional radio receiver shown in FIG. 2 is that the demodulator 211 needs to have a very wide dynamic range. This is caused by the fact that the electric-field strength of the received signal wave and the line quality are monitored and then, the electric-field strength of the received signal wave is controlled or adjusted by the RF switch 203 through the logic circuit 215 according to the monitoring result. Therefore, the level of electric-field strength of the demodulated signal inputted into the demodulator 211 is not kept in a specific level.
For example, if the received signal wave is not amplified by the RF amplifier 204 to suppress the effect of the jamming wave, the electric-field strength of the received signal wave will be lowered compared with the case where no jamming wave exists.